-LRB- CNN -RRB- -- Public executions , death by starvation and torture are common in North Korean political prisoner camps , according to testimony given to human rights group Amnesty International .

Fifteen former inmates and prison guards told Amnesty about their daily experiences in the camps , describing the lack of food , hard labor and cramped conditions .

The camps -- which hold an estimated 200,000 people according to Amnesty -- also appear to be growing in size according to new satellite images obtained by the group .

A satellite image taken of a camp in April was analyzed by the American Association for the Advancement of Science -LRB- AAAS -RRB- against a satellite image taken in 2001 , says Amnesty .

Analysis shows an expansion of the number of buildings in the camp , according to the report .

Amnesty says the facilities occupy vast wilderness sites in the provinces of South Pyongan , South Hamkyung and North Hamkyung and house prisoners accused of criticizing the leadership , those believed to be part of anti-government groups and even those caught listening to South Korean broadcasts .

`` North Korea can no longer deny the undeniable . For decades the authorities have refused to admit to the existence of mass political prison camps , '' said Sam Zarifi , Amnesty International Asia Pacific Director .

`` As North Korea seems to be moving towards a new leader in Kim Jong-un and a period of political instability , the big worry is that the prison camps appear to be growing in size . ''

The report says tens of thousands are believed to be held at one camp simply because one of their relatives has been sent to a camp .

According to testimony , every former inmate at one camp had witnessed a public execution , one child was held for eight months in a cube-like cell so small he could n't move his body and an estimated 40 % of inmates die from malnutrition .

Tortures included placing a plastic bag over the head of a victim and submerging them in water , sleep deprivation , bamboo slivers under the fingernails , and suspending prisoners whose feet and hands have been bound behind them , witnesses said .

One former inmate told Amnesty how he and his father were forced to witness the public execution of his mother and brother , while a former prison guard detailed how inmates would eat snakes , rats and pig feed . One former inmate told how she had picked , cleaned and eaten corn kernels from cow dung .

Other testimony outlined how children at the camps were given minimal education and were often forced into heavy labor , sometimes working until they collapsed .

`` If you fell sick , there were no meals because you did not produce any output , '' the report quoted one witness as saying .

Aidan Foster-Carter , Honorary Senior Research Fellow in Sociology and Modern Korea at Leeds University in England , said : `` It is difficult to get hard data -LRB- on North Korea -RRB- for obvious reasons , and it could be that the numbers of people in the camps are growing . What may be happening is that as part of the changing political situation the government has conducted a purge of people opposed to the succession -LRB- of leader Kim Jong ll by his son Kim Jong-un -RRB- .

`` Entire families have ended up in these concentration camps for political offences . A young boy ended up in one of these camps because his grandfather had expressed political dissent . They are endemic , they are part of the system . ''

The North Korean government -- which has denied the existence of mass political prison camps -- has not publicly commented on the report 's findings .

In its most recent human rights report on North Korea , the U.S. State Department describes the country 's human rights record as `` deplorable . ''

The report cites testimony from NGOs and refugees that claimed camps covered areas as large as 200 square miles and contained mass graves , barracks , worksites and other prison facilities .

`` An NGO reported that one reeducation center was so crowded that prisoners were forced to sleep on top of each other or sitting up , '' the report , updated in March , 2010 , said . `` The same NGO reported that guards at a labor camp stole food brought for inmates by their family members . ''

Journalists Peter Shadbolt and Thair Shaikh contributed to this story .

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Testimony reveals realities of daily life in N Korean political prisoner camps

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Amnesty says the camps could contain as many as 200,000 inmates

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There are fears of a crackdown as the country heads towards a change of leadership